Problem: $ D = \left[\begin{array}{rrr}3 & 2 & -2 \\ 0 & 2 & -1 \\ 0 & 2 & -2\end{array}\right]$ $ A = \left[\begin{array}{rrr}-2 & 1 & -1 \\ -1 & 4 & 0 \\ 2 & -1 & 3\end{array}\right]$ Is $ D- A$ defined?
In order for subtraction of two matrices to be defined, the matrices must have the same dimensions. If $ D$ is of dimension $( m \times  n)$ and $ A$ is of dimension $( p \times  q)$ , then for their difference to be defined: 1. $ m$ (number of rows in $ D$ ) must equal $ p$ (number of rows in $ A$ ) and 2. $ n$ (number of columns in $ D$ ) must equal $ q$ (number of columns in $ A$ Do $ D$ and $ A$ have the same number of rows? Yes Yes No Yes Do $ D$ and $ A$ have the same number of columns? Yes Yes No Yes Since $ D$ has the same dimensions $(3\times3)$ as $ A$ $(3\times3)$, $ D- A$ is defined.